Receivers, Transceivers and Signal Processing
Architectures
Know the superheterodyne principle: mixing converts RF to a fixed intermediate frequency where filtering and gain are easier. The image frequency is separated from the wanted signal by twice the IF, so a higher IF or a preselector improves image rejection. Direct-conversion receivers suffer from LO leakage and audio image effects; SDR architectures digitise early and rely on DSP. Traps include computing the image on the wrong side of the LO and confusing sensitivity with selectivity.
Key parameters
Dynamic range, blocking, intermodulation, reciprocal mixing, noise figure and MDS determine performance in strong-signal environments. AGC controls audio/IF gain to accommodate fading. The noise floor rises with bandwidth: 10 log(B) matters, so doubling bandwidth adds about 3 dB of noise. Know that a narrower filter improves signal-to-noise for CW but may clip SSB audio.
DSP and transceivers
Modern transceivers use DSP for filtering, notch, noise reduction and demodulation. Understand that DSP cannot recover information already lost to front-end overload. Transmit/receive switching, split operation and RIT/XIT are operational concepts frequently examined.
Worked example
A receiver with a 9 MHz IF tuned to 14.200 MHz may have an image at 14.200 − 18 = −3.8 MHz (not useful) if the LO is above; with a 455 kHz IF the image sits 910 kHz away and a preselector can reject it. Always identify LO side first.